SynCore.RTM., sold by Dexter Corporation, Aerospace Materials Division, Pittsburg, Calif. 94565 U.S.A., is a syntactic form film that takes the place of more expensive prepreg plies in stiffness critical structures. This syntactic foam is a composite material consisting of microspheres in a matrix resin. A wide variety of microspheres and matrices can be combined to make SynCore.RTM. materials. Glass is the most common microspheres material of construction, but quartz, phenolic, carbon, thermoplastic and metal coated microspheres have been used. Epoxies curing at 350.degree. F. (177.degree. C.) and 250.degree. F. (121.degree. C.) are the most common matrix resins, but matrices of bismaleimide (BMI), phenolic, polyester, PMR-15 polyimide and acetylene terminated resins have been used to produce SynCore.RTM. products. As a result of the wide variety of materials that successfully make SynCore.RTM. products, they are tailorable to a variety of applications. There is a version of SynCore.RTM. for cocuring with any known available heat-cured composite laminating resins. Syncore.RTM. provides a unique thin film form in isotropic foam structures. Syncore.RTM. allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness limit on honeycomb cores is approximately 0.125 inch. Syncore.RTM. is available in 0.007 to 0.125 inch thicknesses but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet foam can be made thin, but are not drapable and generally require an expensive/heavy adhesive film to bond to the partner composite components. In addition, Syncore.RTM. possess excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. Syncore.RTM. is typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness.
Designing with Syncore.RTM. is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a lighter, stiffer lamination than could be made from prepreg plies alone. Since Syncore.RTM. on a per volume basis typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following:
1. Adding one ply of 0.020 inch Syncore.RTM. and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity. PA0 2. Adding one ply of 0.020 inch Syncore.RTM. and eliminating three plies of prepreg sharply decreases the cost and weight with a small decrease in rigidity. PA0 3. Adding one ply of 0.040 Syncore.RTM. and eliminating three plies of prepreg provides lower weight, cost and sharply increases rigidity. PA0 4. The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the some thickness. PA0 5. A hybrid tape/fabric/Syncore.RTM. construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity.
Syncore.RTM. has been recommended for thin composite structures in any application where flexural stiffness, buckling, or minimum gauge construction is used. It has been shown to save weight and material cost in carbon fiber composites. It has been offered to save weight at approximately the same cost in the case of glass fiber composites.
The manufacturing methods for employing Syncore.RTM. are very similar to those used for prepregs. Because it is not cured, it is tacky and very drapable when warmed to room temperature and is easier to lay-up than a comparable prepreg ply. It can be supplied in supported forms with a light weight scrim to prevent handling damage when it is frozen. It requires cold storage like prepregs, usually 0.degree. F. (-17.7.degree. C.) or below. The various Syncore.RTM. typically have a room temperature out-time that is much longer than their companion prepregs. Because the microspheres provide a large degree of flow control, Syncore.RTM. does not show any unusual migration during cure when normal laminate lay-up and bagging procedures are used. Syncore.RTM. is less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. It will cure void free under full vacuum or low (e.g. about 10 psig.) autoclave pressure. It has been cured at up to about 200 psi without exhibiting sphere crushing.
In a typical application, a sandwich of Syncore.RTM. and prepreg, such as a thicker layer of Syncore.RTM. between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separable molds to obtain various desired shapes.
A method for making a pipe or tube using a prepreg is shown in U.S. Pat. No. 4,289,168. In this patent prepreg is wound in a coil on a mandrel, the mandrel and coil are pushed into a shaping mold, the resin is cured, and the shaped tubular member is removed from the mold and the mandrel is removed from the tube. The tube is formed entirely of prepreg with no syntactic form components such as Syncore.RTM..
It has been found that all syntactic foams, including Syncore.RTM., can absorb water to a volume that exceeds the volume of the resin matrix that makes up the foam. This additional water content in the composite can adversely affect the performance of the composite for a number of applications. The basis for this higher volume of water in the composite has been determined to be associated with the microspheres that are loaded into the resin to form the foam. It has been determined that certain microspheres, over time, gradually absorb water and trap it within the sphere. That added water becomes an incremental weight addition to the cared syntactic form, adding weight to the part with which the syntactic foam composite is associated. In addition, such moisture has the potential of adversely affecting the strength properties of the composite over time.